Contemporary reciprocating engine systems typically employ an encoder system to determine absolute rotary position of an engine. From this absolute rotary position measurement an engine speed signature can be derived. An example of this type of approach is illustrated in FIG. 1. A wheel 101 has teeth, or positional markers 109 disposed radially on an edge of the wheel 101. The wheel 101 is coupled to a crankshaft of the engine and rotates as the crankshaft rotates. In one approach a tooth is missing on the wheel 101 to indicate a fixed absolute position of the wheel. This missing tooth 103 marker is used to synchronize the control of the engine dependent on this fixed position. Teeth 109 on the wheel are positioned a meaningful distance apart--here 20.degree.. An encoder, or position sensor 107 is positioned across from the wheel 101 and is used to sense the teeth 109 as the wheel 101 rotates driven by the engine's combustion process. A signal processing system 111 interprets an output of the encoder 107 and provides a signal 113 indicative of the absolute position of the engine.
Unfortunately, an engine speed signature derived from a digitized target based rotating encoder system has undesirable or erroneous behaviors built into it. Errors include those caused by an error in the physical placement in terms of spacing and/or width of the target teeth measured along a radial axis. If these targets are positioned nonuniformly they can contribute to cause an erroneous representation of true engine position--thus speed. This source of error is typically speed independent. An illustration of a source of this position related error is shown in FIG. 2.
In FIG. 2 the wheel 101 has a first tooth 201 and a second tooth 203 positioned apart from the first tooth 201. Ideally, the position of the second tooth 203, relative to the first tooth 201 is indicated by an angular displacement shown at reference number 207. In this case the angular displacement is too small and is represented by reference number 205. Because of the error associated with the displacement shown by reference number 205 the encoder 107 will produce an erroneous result. This is unfortunate because some combustion misfire systems must rely on an accurate engine speed signature to accurately determine a combustion misfiring event.
Other undesirable behaviors comprise; a speed dependent error associated with a magnetic reaction between a variable reluctance sensor's (used as the encoder 107) housing and a relatively large air-gap associated with the missing tooth 103; and speed dependent dynamic engine effects such as those attributable to piston imbalance and crankshaft twist.
Identified prior art approaches compensated for the encoder profile errors by using a singular profile. This is inadequate because of the dynamic behavior of the encoder profile error over the operating speed range of the engine. Furthermore the known prior art approaches acquired the profile under non-fueled conditions. This approach was taken to assure that errors in encoder position--thus speed measurement associated with a misfiring combustion process. Additionally, these prior art schemes could not safely de-fuel the engine running at high speed, particularly at high load, because it posed a safety hazard or unacceptable driveability behaviors. Because of this the associated misfire detection system had to be disabled at high engine speeds because the encoder profile correction was inadequate to improve the signal fidelity enough to reliably detect misfiring. This is because engine combustion misfire detection systems that used the single profile correction scheme were incapable of distinguishing a misfire from the background noise in the encoder derived engine speed signal at high engine speeds.
FIG. 3 is a chart illustrating a high fidelity combustion torque dependent acceleration waveform over many engine cycles with a 1-in-25-cylinder induced misfire event. The acceleration waveform is derived from velocity data as the engine crankshaft rotates. This waveform has minimal encoder profile errors in it. The main part of the waveform 301 represents a noise portion and the strong negative going portion 303 represents the induced misfire. This waveform is considered relatively high fidelity because an amplitude of the noise portion is significantly smaller than an amplitude of the strong negative going portion 303 and thus the portions 301 and 303 are easily distinguishable.
FIGS. 4A-4D are charts illustrating combustion torque dependent acceleration waveforms at 6,000 RPM with a 1-in-25-cylinder induced misfire event. FIG. 4A shows an example of a combustion torque dependent acceleration waveform without the benefit of any encoder profile correction. Comparing this to the waveform in FIG. 3 it can be readily appreciated that the 1-in-25-cylinder induced misfire events are masked by systemic noise related to the various causes of error introduced above.
FIGS. 5A and 5B show timing diagrams illustrating engine speed variability dependent on encoder profile error. FIG. 5A shows an encoder output at a fixed shaft RPM with an ideal waveform. FIG 5B shows an encoder output at the same fixed shaft RPM, but having a profile. In this case downward-going edges are not perfectly monotonic. Shaft speed calculated from one downward-going edge to the next will be inaccurate due to the encoder profile errors.
What is needed is an improved approach for encoder profile correction for rotating position encoders in reciprocating engines that eliminates the various erroneous behaviors associated with such encoders.